1. Field of the Invention
The present invention relates to a process and composition for arsenic removal from ground water.
2. Description of the Related Art
Arsenic is a nonmetallic element present in nature and comprises 0.00005% of the earth""s crust in organic or inorganic form. The principal process of dispersion of arsenic in the environment is by water. Even with sedimentation, the solubility of arsenates and arsenates is sufficient for this element to be transported in water systems. The concentration of arsenic in fresh natural waters varies widely and generally depends on the forms of arsenic in the local soil.
Exposure to arsenic has been linked to alterations in the skin and organs of the respiratory, gastrointestinal, cardiovascular, nervous, and hematopoietic systems. Chronic exposure can cause the accumulation of arsenic primarily in the bones, muscles, and skin, and to a lesser extent in the liver and kidneys. A condition that commonly results from prolonged exposure to inorganic arsenic via drinking water or drugs is palmarxe2x80x94plantar hyperkeratosis.
The ingestion of water having a high arsenic content usually results in chronic poisoning, whose primary manifestation is skin pigmentation and calluses on the palms of the hands and soles of the feet. Arsenic compounds generally cause relaxation of cutaneous capillaries. Dilation of the cutaneous capillaries, in turn, produces secondary effects on the circulatory and nervous systems.
Studies with laboratory animals indicate that trivalent inorganic arsenic is more toxic than the pentavalent form because pentavalent compounds have less of an effect on enzymatic activities. However, pentavalent compounds can be reduced to more toxic trivalent compounds in vivo.
Arsenic is eliminated from the body naturally through urine, feces, sweat, and exfoliation of the epithelium (desquamation).
In general, drinking water treatment is focused on removing color, turbidity, and microorganisms of fecal origin. This goal may generally be achieved through a combination of processes such as coagulation, flocculation, sedimentation, filtration, and disinfection. The removal of chemical elements such as arsenic from water, however, requires resort to more sophisticated processes. In developed countries, reverse osmosis, ion exchange, and activated carbon are conventional techniques used in purifying water in large agglomerations.
However, the above conventional techniques for removing impurities, such as arsenic from ground water, are generally prohibitive or unavailable to small populations living in remote dwellings.
Reactions that lead to the removal of dissolved arsenic include adsorption with clay or co-precipitation with precipitated metallic ions.
Arsenic removal using conventional coagulation treatment and softening with soda frequently depends on the pH of the water treated, on the type and dose of coagulant, and on the initial concentration of the arsenic. Of these variables, the most important is the pH of the water to be treated.
In water, pentavalent arsenic can be effectively removed by coagulation with aluminum sulfate or ferric chloride and by a softening processes employing lime and soda. When the concentration of arsenic in untreated water exceeds 1.0 mg/l, the removal of arsenic decreases as the initial concentration increases, particularly if aluminum sulfate is used.
Coagulation with aluminum sulfate or ferric chloride does not remove trivalent arsenic as efficiently as in the case of pentavalent arsenic. As+3 could be removed from water using conventional coagulation and/or softening with lime and/or soda, if prior to treatment the As+3 is oxidized to As+5. For example, conducting a prechlorination process prior to conventional treatment provides oxidation of As+3 to As+5 and then removal using conventional techniques for removing As+5.
One significant health problem facing people in less developed countries is the ingestion of arsenic through drinking water. Therefore, one object of the present invention is to provide a simple process and composition to remove arsenic naturally present in ground water.
Another object of the present invention is to provide a low-cost process and composition to remove the arsenic naturally present in ground water used for human consumption in rural areas and less developed countries.
Yet another object of the invention is to provide a process and composition to remove the arsenic naturally present in groundwater used for human consumption whose efficiency can be replicated under varying conditions of water quality, application, geography, and climate.
In a first aspect, the invention provides a process for removing arsenic from ground water comprising:
(a) contacting a clay, a coagulant, and an oxidizer with water containing arsenic to form a coagulated colloidal mixture;
(b) adsorbing arsenic from the water onto the coagulated colloidal mixture; and
(c) separating the water from the coagulated colloidal mixture.
Optionally, the process may include first disposing the arsenic-containing water in a container and agitating the material to disperse the colloidal mixture.
A second aspect of the invention provides a composition for arsenic removal from ground water comprising a clay, a coagulant, and an oxidizer. Preferred compositions include activated bentonite as the clay, ferric chloride or aluminum sulfate as the coagulant, and chlorine as the oxidizer. The most preferred compositions have a mass ratio between the clay, the coagulant and the oxidizer between about 1000/160/10 and 1000/50/5, and more preferably, the ratio is about 1000/60/5, when ferric chloride is the coagulant. When the coagulant is aluminum sulfate, the most preferred mass ratio between the clay, the coagulant and the oxidizer is between about 1000/160/10 and 1000/30/5, and more preferably, the ratio is about 500/50/5. The composition is preferably in the form of a solid homogeneous mixture.